The present invention relates to a gas sensor such as an oxygen sensor, HC sensor and NOx sensor.
An example of a gas sensor includes a rod-shaped or tube-shaped sensor element having a detecting section for detecting a gas component at a tip end thereof, and a metal casing in which the sensor element is disposed. Such a gas sensor is attached to a predetermined attaching portion such as an exhaust pipe by means of a threaded portion formed on an outer periphery of a housing which forms part of the casing. The sensor element partially protrudes from the casing so as to allow the detecting section to be held in a gas to be measured. In many gas sensors, there is provided a protector for covering the detecting section and thereby protecting the detecting section from water and toxic substances. The protector has a peripheral wall formed with gas holes through which a gas to be measured such as exhaust gas is introduced into the protector for contact with the detecting section. In this connection, recently, in order to make higher the ability to protect the detecting section, a dual-walled protector having inner and outer protector members is widely used.
While the above described dual-walled protector can make higher the ability to protect the detecting section, the resistance to flow of the gas to be measured through the gas holes increases due to the dual-walled structure. In many cases, such an increase in flow resistance causes the speed with which the gas to be measured is exchanged between the inside and outside of the protector to become lower. For this reason, in case a concentration of a gas component to be measured changes rapidly, there is possibly caused a disadvantage, for example, that measurement of an air-fuel ratio lacks in accuracy.
The above described disadvantage will often become a serious problem when the gas sensor is a xcex-type oxygen sensor. The xcex-type oxygen sensor includes a detecting section constituted by an oxygen concentration cell element. The oxygen concentration cell element consists of a layer of oxygen ion conductive solid electrolyte such as zirconia and porous electrodes formed on the opposite sides of the solid electrolytic layer. The xcex-type oxygen sensor detects an oxygen concentration on the basis of a variation of the electromotive force of the oxygen concentration cell element. The xcex-type oxygen sensor can be responsive relatively sharply to a rise of the oxygen concentration, i.e., transition of the gas to be measured from a lean condition containing oxygen excessively to a rich condition containing combustible components excessively since the combustible components are adsorbed to the porous electrodes rapidly. However, in case of transition from the rich condition to the lean condition, a responsive delay in response to a fall of the oxygen concentration is liable to be caused since desorption of the combustible gas components from the porous electrodes can not be attained so rapidly as the adsorption thereof and in addition for the reason of the above described delay in exchange of the gas between the inside and outside of the protector.
Further, a responsive delay in response to a fall of the oxygen concentration will cause the following problem. An engine control unit performs such a combustion control that shifts the gas to be measured to a rich condition side (i.e., a combustion control that causes an intake mixture to shift to a rich side) when the output of the oxygen sensor falls to a certain constant level. However, since the output of the sensor is not lowered until the gas to be measured is put into a considerably lean condition, the combustion control for shifting the gas to be measured to a rich condition side is inevitably delayed. When the combustion control for shifting the gas to be measured to a rich condition side starts at long last, the output of the sensor is caused to rise relatively sharply. In response to this, the engine control unit stops the combustion control before the gas to be measured is put into a sufficiently rich condition (i.e., before an intake mixture becomes sufficiently rich). As a result, the gas to be measured is controlled under a condition of being shifted to a lean condition side at all times. Namely, a so-called lean shift control, i.e., a control for causing the gas to be measured to shift to a lean condition side tends to be performed. This can be regarded as a disadvantage caused by the fact that the behavior of the sensor at the time of a rise of its output is not equivalent to that at the time of a fall of its output.
It is accordingly an object of the present invention to provide a gas sensor which has an improved responsiveness, particularly at the time of a fall of its output.
It is a further object of the present invention to provide a gas sensor of the foregoing character which can decrease the difference between the responsiveness at the time when its output rises and the responsiveness at the time when its output falls.
It is a further object of the present invention to provide a gas sensor of the foregoing character which is particularly suited for use as an air-fuel ratio sensor for automotive vehicles and is therefore particularly useful from an anti-pollution preventing point of view.
To accomplish the above objects, there is provided according to an aspect of the present invention a gas sensor which comprises a sensor element having at a tip end portion thereof a detecting section for detecting a component of a gas to be measured, a casing accommodating therewithin the sensor element and having an open end portion from which the detecting section protrudes, and a protector attached to the open end portion of the casing for covering the detecting section. The protector is cup-shaped and has a bottom wall and a circumferential wall. The circumferential wall is dual-walled and includes an inner circumferential wall section and an outer circumferential wall section. The outer circumferential wall section of the protector has a first gas hole. The inner circumferential wall section of the protector has a second gas hole. The bottom wall of the protector has a bottom gas hole. When the direction of the tip end portion of the sensor element is referred to as forward, the first gas hole is disposed forward of the second gas hole and a front end of the sensor element is disposed forward of the second gas hole such that a gas to be measured is caused to flow rearward within a space between the inner and outer circumferential wall sections and forward within a space around the detecting section.
According to another aspect of the present invention, there is provided a gas sensor which comprises a sensor element having at a tip end portion thereof a detecting section for detecting a component of a gas to be measured, a casing accommodating therewithin the sensor element and having an open end portion from which the detecting section protrudes, and a cup-shaped protector attached to the open end portion of the housing for covering the detecting section. The protector is dual-walled and has cup-shaped inner and outer protector members which are bottomed. The inner and outer protector members have circumferential walls between which a predetermined space is defined. The space is determined so as to satisfy a relation of g1 greater than g2 where g1 is the distance between an inner surface of the circumferential wall of the outer protector member and an outer surface of the circumferential wall of the inner protector member and g2 is the distance between an inner surface of a bottom wall of the outer protector member and an outer surface of a bottom wall of the inner protector member. The outer protector member has at the circumferential wall thereof a plurality of first gas holes. The inner protector member has at the circumferential wall thereof a plurality of second gas holes. When the direction of the tip end portion of the sensor element is referred to as forward, the first gas holes are disposed forward of the second gas holes. A front end of the sensor element is disposed forward of the second gas holes. The inner and outer protector members have at bottom walls thereof bottom gas holes.